In radio communication systems, the AMC (Adaptive Modulation and Coding) technique is adopted as a transmission scheme for high-speed packet transmission for changing a modulation scheme and coding rate adaptively in response to fluctuation of propagation environment.
The AMC technique will be explained briefly below. First, downlink quality is measured all the time in a communication terminal apparatus (hereinafter “UE” which means user equipment), and transmits measurement results (with downlink quality information) to a base station apparatus (hereinafter “Node B”) in uplink. Node B selects a MCS (Modulation and Coding Scheme), which is a combination of a modulation scheme and a coding rate, based on downlink quality information and transmits downlink packets using the selected MCS.
Patent Document 1 discloses an improved AMC technique. Patent Document 1 discloses techniques of: dividing transmission schemes (i.e. modulation schemes) into a plurality of groups; deciding in which group a selected transmission scheme is included; transmitting group information when the group information is changed; and transmitting information that specifies the transmission scheme from the group at regular intervals.
In addition, Patent Document 2 discloses another improved AMC technique. Patent Document 2 discloses a technique of reporting downlink quality information in a long period (long cycle) than the TTI (Transmission Time Interval), selecting a communication terminal preliminarily based on the report, and further reporting downlink quality information on the preliminarily-selected communication terminal in a short cycle.
There may be a technique described below, provided by combining the technique disclosed in above-described Patent Document 1 and the technique disclosed in above-described Patent Document 2. In this case, CQI is used for downlink quality information to be transmitted from UE to Node B.
FIG. 1 illustrates a CQI table. In this figure, there are 24 levels of CQI's and these 24 levels are divided into six groups per four levels each. These groups are specified by six higher CQI levels and the four levels in each group are specified by four lower CQI levels. For example, when the CQI level is 7, the higher CQI level is 2 and the lower CQI level is 3. In this case, the matching modulation scheme is QPSK (with a repetition factor of four), coding rate R is 7/16, and the number of information bits is 700.
Here, the number of information bits is the number of data bits not yet subjected to coding to be actually transmitted. For example, the CQI of level 1 and the CQI of level 2 have the same number of bits after coding, 800 bits, but have different numbers of bits before coding, 100 bits and 200 bits, respectively. Further, in this case, the higher CQI is reported in a long cycle and the lower CQI is reported in a short cycle.
Next, a case will be described below using FIGS. 2A and 2B where CQI is reported to Node B using the CQI table shown in FIG. 1. FIG. 2A illustrates changes of received SIR (Signal to Interference Ratio) in UE over time, where the vertical axis is SIR and the horizontal axis is time. At time t0, the SIR is 5.5 dB and the CQI level is 6, so that the higher CQI of 2 and the lower CQI of 2 are reported. Likewise, at time t1, the SIR is 4.5 dB and the CQI level is 5, so that the lower CQI of 1 is reported.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-261851    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-320262